A satellite based radio ranging system allows a user to precisely determine position and time by passively receiving the orbiting satellites' transmission of ranging signals and navigation data consisting essentially of each satellite's ephemeris (i.e., position) and clock (time of the day) information, ionospheric modeling parameters, and other status information. The United States' Global Positioning System (GPS) and the Russian Global Navigation System (GLONASS) are such radio navigation systems. For GPS, the navigation data has a data rate of 50 bits per second.
The user's receiver, by obtaining the arrival times of the signals transmitted by the satellites in his field of view, which are precisely measured relative to the receiver's own clock, obtains its distances (ranges) to the respective satellites to within a constant bias. The value of the constant bias is equal to the difference between the satellite's and the receiver's clock times. Since the satellites' clocks are synchronized with system time, this constant is the same for all satellites. The distances thus measured are called pseudoranges since they are offset from the actual distances by a constant value which is the same for all satellites tracked. With four or more satellites in view, the four unknowns consisting of the user's position (longitude, latitude, and elevation) and the user's clock offset with respect to system time can be solved using the measured user-to-satellite pseudoranges, and the satellites' ephemeris in the navigation data broadcast by the satellite.
In order to allow the user to measure pseudoranges, the satellites' ranging signals are wide band pseudorandom noise (PRN) coded signals which are radio frequency (RF) carriers modulated by a wide band PRN code, modulo-2 added to the navigation data. Unique PRN codes are used for different satellites, and different types of PRN codes with different chip rates are used for different system applications. In GPS, for example, a 1 MHz chip rate C/A code is used for initial acquisition and less accurate civilian applications, while a 10 MHz chip rate P-code is used for higher accuracy government applications. The PRN codes are designed such that there are minimum cross-correlation between different PRN codes. For example, the GPS C/A codes are 1023 chip length Gold codes which have a maximum periodic cross-correlation value of only 65. This design feature allows the user receiver to separate the ranging signals received from a number of GPS satellites by acquiring and tracking the unique PRN codes of the GPS satellites even though they are transmitted on the same RF frequency (1575.42 MHz for GPS L1, 1227.6 MHz for GPS L2).
A common circuit used to track PRN signals is the delay lock loop which correlates early, punctual, and late versions of the locally generated PRN code against the received signal and obtains an estimate of the time difference between the local code and the received code from the difference between the correlation of the early version of the local code against the received signal and the correlation of the late version of the local code against the received signal.
The accuracy of the user position and time determination of a satellite based radio navigation system is affected by several factors: the accuracy of the satellite ephemeris and clock time given in the satellite broadcast navigation message, the propagation delays introduced by the ionosphere and the troposphere, receiver noise and quantization effects, radio frequency interferences, multipath fading, and the relative geometry of the satellites and the user, which is measured in terms of geometric dilution of precision. Some of these error sources, in particular, the satellite ephemeris and clock errors, the ionospheric and tropospheric delays, can be eliminated in a differential GPS (DGPS) system, which can either be local area differential or wide area differential. In local area differential systems the errors in the determined position of the reference receiver, which is located at a precisely surveyed site, are subtracted from the position solution of the user. For such DGPS applications the user is usually within 10 km of the reference receiver. Under these circumstances the error caused by satellite ephemeris and clock and by ionospheric and tropospheric delays are almost identical at both the user and the reference receiver, and are thus practically canceled in the correction process. The remaining major error sources are then receiver noise, quantization effects, interference and multipath fading. These effects are uncorrelated between the reference and user receivers, and are not canceled from each other in the correction process. To alleviate these effect, a wide area DGPS system (e.g., the FAA's Wide Area Augmentation System) broadcasts correction signals from geostationary satellites to the users. The correction signals are derived from a network of ground reference stations that estimates the satellites' fast clock errors, slow clock and ephemeris errors and ionospheric grid point vertical delays. Similar to the local area DGPS systems, most of the error sources are mitigated in the correction process except for receiver noise, quantization noise, interference and multipath fading. The effects of receiver thermal noise and quantization noise can be reduced with averaging. Interferences can be mitigated with frequency management and regulation. Multipath fading thus becomes the most detrimental error source in a differential GPS system. Sub-meter accuracy can generally be obtained by differential systems if there is no multipath effect. Multipath error in C/A code PRN tracking with early minus late discriminators, on the other hand, can be as big as one chip and is usually several meters in magnitude. Multipath rejection is thus an important design objective in high quality PRN receivers.
The number of multipath signals, their relative delays and RF phase offsets with respect to the direct path signal are all functions of the satellite to user antenna geometry relative to reflecting objects around the receiver antenna. Since multipath signals always travel a longer distance than the direct path signal, they are invariably delayed with respect to the direct path signal and will suffer a loss in power in the reflection process. If the multipath has a delay in excess of one PRN chip in time with respect to the direct path signal, it will not correlate with the local code and will not affect the pseudorange measurement accuracy once the delay lock loop is locked onto the direct path signal. However, if the multipath delay with respect to the direct path signal is within one chip in time, the error signal measuring the relative time offset between the local code and the received signal in an early-minus late discriminator configuration is usually biased by the multipath. For C/A code receivers this problem is significant since the C/A code chip time is 1 microsecond in length, allowing multipaths delayed with respect to the direct path signal by as much as 300 meters to affect the pseudorange measurement accuracy. In addition, since the chip time is 300 meters in length, multipath error, even in a small fraction of a C/A chip, can be very detrimental. Multipath error is thus one of the most troublesome problems faced by PRN receiver designers.
The typical ways used to mitigate multipath effects are: (1) careful site selection and antenna design to modify the gain pattern of the user antenna to reduce the magnitudes of the antenna gain at low elevation angles to eliminate multipath signals reflected by surrounding, low altitude (compared to the satellite) objects; and (2) by receiver processing. Large ground planes and choke ring antennas have been used with some success in the past. However, at L-Band the choke ring antenna is fairly large in diameter, and large ground planes are not always practical on all installations. Receiver processing to reduce multipaths can be implemented in two different ways: (a) by narrowing the spacing of the early minus late correlator (see U.S. Pat. No. 5,495,499, Feb. 27, 1996, Fenton et al.; Van Dierendonck et al, "Theory and Performance of Narrow Correlator Spacing in a GPS Receiver", Navigation: Journal of the Institute of Navigation, vol. 39, No.3 (Fall 1992); and L. Hagerman, "Effects of Multipath on Coherent and Noncoherent PRN Ranging Receivers", Aerospace Corporation Report No. TOR-0073(3020-03)-3 (May 15, 1973)); and (b) by estimating the correlation function parameters that vary with multipath distortion--the parameters of interest discernible from an estimate of the shape of the autocorrelation peak include the direct signal path time and phase offset (see U.S. Pat. No. 5,414,729, May 9, 1995, Fenton). Although approach (a) has a smaller multipath error compared to the conventional early minus late correlator with one chip spacing, it still exhibits a significant non-zero multipath error for multipath delays of up to one chip with respect to the direct signal. Approach (b) has the drawback of a relatively high hardware implementation complexity and cost, and may experience large errors before the numerical computation converges to a correct solution.
FIG. 1 shows the error envelope of an early minus late correlator with one chip and 0.2 chip spacings, respectively, caused by a specular multipath at half of the signal's amplitude and at various delays with respect to the direct signal of up to and exceeding one chip. The upper curves 201, 202 (with positive pseudorange errors) correspond to the cases where the multipath is in RF phase with the direct signal. The lower curves 203, 204 (with negative pseudorange errors) correspond to the cases where the multipath is 180 degrees out of RF phase with respect to the direct signal. The computed maximum pseudorange error of the conventional one-chip early minus late correlator is 0.25 chips (75 meters) with this multipath amplitude. The maximum error of the 0.2 chip spacing early minus late correlator is about 0.05 chips (15 meters) with the same multipath amplitude. In the narrow spacing early minus late correlator the maximum pseudorange error does not decrease before the multipath delay is larger than one chip. This means that multipaths at varying delays for up to one chip can result in the maximum pseudorange error which can only be reduced by narrowing the correlator spacing. The results in FIG. 2 demonstrate that some improvement in the multipath rejection capability of the early minus late type of correlators will be desirable and also necessary.
What is needed is a way to reduce the pseudorange measurement error of a PRN receiver, especially those of the low chip rate C/A code type, in the presence of multipath distortion. The desired method of implementation should be relatively low cost and simple in hardware complexity, and able to eliminate pseudorange errors caused by multipaths with delays with respect to the direct signal exceeding a small fraction of a PRN chip (e.g., 0.05 to 0.01 chips, or 15 to 30 meters in the case of C/A code receivers). The desired implementation should not degrade signal acquisition capability of the receiver, should be able to operate in both the coherent and noncoherent modes, and should not require special antenna designs or site selection.